Concern over personal security has led to the development of various alarm devices for indicating the unauthorized entry into a residence. With a permanently occupied home, it is conventional to install a permanent alarm system connected to all doors and windows. This may be used alone or in conjunction with an interior alarm system such an a motion sensor. These permanent systems are not suitable for use in temporary premises such as hotel rooms and many apartments. The primary concern with such premises is an unauthorized entry through the door, since windows are often inaccessible or permanently closed. Potential solutions to the problem are described in U.S. Pat. Nos. 1,305,190, 4,339,747, 4,575,713.
In U.S. Pat. No. 1,305,190, there is described an alarm that may be suspended from a doorknob and which has a spring loaded switch that may be held in an electrically open condition by inserting a pair of spring blades between the door and door frame. When the door is opened, the blades separate and the switch contact is closed, thus activating the alarm. With this device, both the switch and the alarm mechanism are accessible to an intruder so that the alarm may very quickly be silenced before an adequate alert has been given.
U.S. Pat. No. 4,339,747 describes an alarm that is mounted on a doorknob and incorporates a pendulum switch that activates an alarm when the knob is turned. The major drawback of this device is its complete enclosure of the doorknob when installed. This means that the doorknob cannot be used in the normal way without fully removing the alarm. Additionally, because the alarm must be removed to grasp the door knob, its removal is made very simple so that an intruder could simply remove the alarm from the doorknob, giving the opportunity to silence it.
U.S. Pat. No. 4,575,713 describes another pendulum type alarm. In this case, the alarm is suspended from a doorknob using a flexible strap. Again, the alarm could readily be removed from the doorknob and silenced by an intruder.
The aim of the present invention is to provide an improved portable alarm suitable for use in hotel rooms and the like.